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Borrowings - Securitizations (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 28, 2017 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Nov. 30, 2017 | Mar. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  |  |  |  |  |  | $ 1,110,000,000 | $ 1,900,500,000 | $ 1,448,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization |  |  |  |  |  |  | $ 1,208,963,000 | $ 2,101,644,000 | $ 1,606,804,000
2017-A |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued | $ 772,000,000 |  |  |  |  |  |  |  |
Weighted Average Life | 4 years 3 months 7 days |  |  |  |  |  |  |  |
Basis spread on variable rate | 0.93% |  |  |  |  |  |  |  |
2017-B |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  |  |  |  | $ 676,000,000 |  |  |  |
Weighted Average Life |  |  |  |  | 4 years 25 days |  |  |  |
Basis spread on variable rate |  |  |  |  | 0.80% |  |  |  |
2018-A |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  |  |  | $ 670,000,000 |  |  |  |  |
Weighted Average Life |  |  |  | 4 years 5 months 4 days |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.78% |  |  |  |  |
2018-B |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  |  | $ 686,500,000 |  |  |  |  |  |
Weighted Average Life |  |  | 4 years 4 months 24 days |  |  |  |  |  |
Basis spread on variable rate |  |  | 0.76% |  |  |  |  |  |
2018-C |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  | $ 544,000,000 |  |  |  |  |  |  |
Weighted Average Life |  | 4 years 3 months 25 days |  |  |  |  |  |  |
Basis spread on variable rate |  | 0.77% |  |  |  |  |  |  |
2019-A |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Total Issued |  |  |  |  |  | $ 453,000,000 |  |  |
Weighted Average Life |  |  |  |  |  | 4 years 3 months 3 days |  |  |
Basis spread on variable rate |  |  |  |  |  | 0.92% |  |  |
2019-B |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  |  | 0.00% |  |

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